a) Field of the Invention
The invention is directed to an objective for telescope-type stereomicroscopes with magnification that is adjustable at post-magnification systems.
b) Description of the Related Art
In this type of stereomicroscope construction, two separate light bundles are guided through one and the same objective in order to achieve the stereoscopic impression. As a result, the lens diameters of an objective of this kind are significantly larger than those of objectives for monoscopic examination of objects or in Greenough-type stereomicroscopes.
The objectives are generally used in connection with post-magnification systems by which the microscope magnification can be preselected. On the side of the objective remote of the object, the two light bundles enter the post-magnification systems so as to be oriented parallel to one another. The entrance pupils of the two post-magnification systems have a distance B from one another which is defined as the stereo basis. Provided these entrance pupils do not penetrate one another, the following relationships generally apply:DAP≧B+DEP  (1)DEP≦B,  (2)where DAP is the diameter of the exit pupils at the objective and DEP is the diameter of the entrance pupils at the post-magnification systems. When the condition for the required magnification is maintained in a stereomicroscope of this constructional type, the diameter of the entrance pupils DEP is made use of to the fullest extent at maximum magnification.
The light coming from different points on the object enters the post-magnification systems at different angles. This angle ω, usually referred to as the field angle, at which the light coming, e.g., from an off-axis point P on the object enters the associated post-magnification system, depends on the selected magnification. The angle ω has its maximum at the lowest possible magnification setting.
In order to make it possible for customers to use microscopes at magnifications which allow overview imaging of an object as well as detailed viewing without having to change objectives, objectives are required in which there is more space available for suitable post-magnification systems and which further offer a larger usable aperture in the object space. Further, the requirements for objectives have become increasingly demanding with respect to correction of chromatic aberration, image field flattening, and absence of distortion.
U.S. Pat. No. 6,271,971 describes an objective for telescope-type stereomicroscopes with the aim of optimizing the ratio of the required mounting space to the objective focal length.
JP 2001-147378 describes the construction of objectives which are provided for use in telescope-type stereomicroscopes. Considered from the microscope body, they comprise a first lens group with positive refractive power, a second lens group containing at least one cemented triplet, and a third lens group with positive refractive power. The term lens group includes individual lenses, cemented components or combinations thereof.
JP 2001-21955 shows objectives of a different constructional type which are likewise suitable for telescope-type stereomicroscopes. The objectives proposed therein comprise, considered from the microscope body, a first lens group with positive refractive power containing a biconvex cemented component and a second lens group containing at least two cemented components. In this case also, the term lens group includes individual lenses, cemented components and combinations thereof.
In the objectives described in the above-cited publications, correction of the chromatic aberration, field flattening and distortion is successful only conditionally. The demands for more mounting space for the post-magnification systems and larger usable apertures in the object space are also not met with respect to current requirements.